1. Field of the Invention
The present invention relates to hydraulic gear pumps and motors, and, more particularly, to high pressure gear pumps of the type having a drive pinion cooperating with a surrounding internal gear ring and an intermediate filler member.
2. Description of the Prior Art
High pressure hydraulic gear pumps of the internally geared type have been known for some time. Such a gear pump may have a filler member of sickle-shaped outline, or one or two semi-sickle-shaped or curved-wedge-shaped members.
In operation, the three basic constituent members of the gear pump, pinion, internal gear ring, and filler member, are subjected to friction under pressure and consequently undergo uneven operating wear. This uneven wear results from the directional nature of the hydraulic pressures which are generated inside the hydraulic pump or motor in the axial as well as radial sense.
The prior art in this field already contains various suggestions aimed at reducing the operating wear by arranging compensating pressure fields inside the gear pump through which the directional pressures are fully neutralized or almost fully neutralized. It is known, for example, that the axial pressures can be compensated for by arranging on both sides of the gears axially movable pressure plates which have arranged on their outer sides suitable compensation pressure fields of predetermined shape which are subjected to the operating pressure of the hydraulic pump or motor.
Several other prior art suggestions concern themselves with the compensation of the radial pressures, especially the pressure to which the rotating internal gear ring is subjected. These solutions include three basic approaches:
(A) The internal gear ring is radially displaceable and its displacement is controlled by a control piston, or by one or more pressure compensation fields on the outer periphery of the internal gear ring, whereby the latter is pushed radially inwardly against the filler member and the pinion;
(B) The filler member is a curved wedge and its supporting pin is displaceable in the circumferential sense, the displacement of the pin being controlled by means of a compensation piston, so that the filler wedge is advanced into sealing contact with the converging teeth of the pinion and internal gear ring;
(C) The drive pinion is displaceable in a radial direction, the displacement of the pinion being controlled by hydraulic compensation pistons which engage the shaft extensions of the pinion, thereby pushing the pinion radially against the filler member.
A common shortcoming of these pressure compensation approaches is their design complexity, which reflects itself in a need for very close production tolerances, with attendant assembly problems and consequent high production costs. A further common disadvantage of these prior art hydraulic pumps and motors is that their design complexity reflects itself in greater overall space requirements.